The Koran burnings have underscored the deep cultural divide that still exists in Afghanistan more than 10 years after US troops invaded to oust the Taliban and has deepened public mistrust of Western troops struggling to stabilise the country.

If the shootings are tied to Afghan forces, questions will also arise about Taliban infiltration as well as their ability to secure Afghanistan once NATO combat forces withdraw in 2014.

Taliban spokesman Zabihullah Mujahid said in a statement four high-ranking Americans had been killed, though the Islamist group often exaggerate and inflate claims of casualties.

"The attack came from the Mujahidin of the Islamic Emirate in revenge for the American soldiers' repeated desecration of our religion, especially the latest intentional incident in the Bagram airfield which they burnt Korans," he added, using another name the group call themselves.